Ask Us

Our knowledge of the Universe is admittedly limited, but the scientific method works by taking what observations we have, making theories, and then testing them. In the testing, we learn a little more about the Universe -- whether the theory was right or wrong. I like to think of the Universe as an enormous puzzle, and scientists are trying to put the pieces together. We don't have all the pieces, and don't know where all the pieces we have fit into the puzzle.

This may limit furthur investigation in areas that conflict with our scientific knowledge (like pseudoscience, such as astrology and ESP), but until there are repeatable and measurable results from those fields, most scientists correctly ignore them.

How can we make any conclusions about what is beyond our observation? On that hypothetical grain of sand, understanding the physics of that limited area may help us to see or travel furthur, so we can find out more about the larger Universe.

Dr. Eric Christian

Do what we all do - write down your theory and send it to any of the various scientific journals. Their job is exactly to peer review new work.

We respect your work and interest, but as scientists (and as scientists supported by taxpayers), we have to be conservative in accepting novel views, sometimes to the detriment and progress of science; you see, we have to accept new theories only when absolutely forced upon us. Until then, we work within the framework of the old theories, tinkering with them to try fit them to the existing body of evidence presented by the observations. It is true that progress is slower this way, but it is more certain. On rare occassions, a "revolution" may occur, but they are truly extraordinary. Until you can point to an observation that your theory explains and that the scientific community hasn't explained, you haven't made any progress.

You might also look at the Imagine the Universe! answer about getting a new theory published.

Drs. Louis Barbier and Eric Christian
(June 2000)

A PhD is required to be taken seriously in this field. It also gives you contacts on whom you can try out your ideas. Go to school and learn as much as you can!

Dr. Louis Barbier and Beth Barbier

It's wonderful that you are interested in NASA's work, and we hope that our site helps to further that interest. But please remember that, while NASA isn't perfect, it is full of highly educated and experienced people with very difficult, complex jobs. To design and build a spacecraft, for example, one must consider many constraints -- money, weight, time, environmental conditions, etc.

If you are interested in pursuing a career to work for NASA, consider directing your education toward science, math, physics, or engineering.

Beth Barbier and Dr. Eric Christian

I did a Google search for [satellite imagery earth "real time"], which returned 1.5 million sites. Looking through the first page, there are several that look promising:

• Earth and Moon Viewer

• Satellite Composite Images from the University of Wisconsin - Madison

and of course:

• Google Earth

I can also recommend another site that your son may find appealing, though it doesn't have real-time imagery. StarChild is designed for students under age 14.

I can also suggest that you check the list of links on our site, especially the specific science topics, spacecraft, and space pictures sections. You might find something there that interests him.

And there's always the very popular Astronomy Picture of the Day.

I hope your son will find something in this list that draws him in.

Beth Barbier
(October 2005)

These NASA Web-based learning centers are a good place to start:

Dr. Eric Christian

On this site, you can check out the interview with Dr. Louis Barbier, and there's a biography of Dr. Eric Christian at the Imagine the Universe! site. You can also see Dr. Christian's home page. Drs. Christian and Barbier are cosmic ray astrophysicists at NASA GSFC.

You can find more information about careers in astronomy and astrophysics on the Imagine the Universe! site.

Ms. Beth Barbier

You can find out what scientists do at the NASA Goddard Space Flight Center's Astrophysics Science Division by reading through the Featured Scientist biographies at our sister site, Imagine the Universe!

Beth Barbier
(April 2004)

In a word, "No." However, isn't this the way we all learn and find new ideas? I find there is nothing harder to teach a student of science than to encourage them to associate what they know with what they don't in an effort to find new understanding insights. The pursuit of science is so far from a linear and orderly progression that I sometimes worry that textbooks knock all the creativity and curiosity from young people.

In fact, I have a student working with me on a hard project and we have gone down so many blind alleys as we work to the finish that I am surprised he has stayed with me. I was discussing this with a colleague also working on the project and we agreed that the best thing a student can learn from a professor is that we are also learning and making mistakes. As she said, "Then I guess he's learned a lot from us!" and she's right.

Remember, the best thing I can teach you is that we are all students, and I make a lot of mistakes.

Dr. Charles Smith
(January 2006)

It is a great idea to try to get experience doing "hands-on" research during the summer. There are a number of programs offering summer internships both here at Goddard and also at various universities. Here are a few that you might want to look up:

• The National Space Club at Goddard
• SURF at Caltech

You might want to discuss your options for research with faculty at your university as well.

Dr. Georgia de Nolfo
(November 2002)

We are almost always working on several "open" problems at the same time, as there are always interesting and intriguing observations and paradoxes that have not been satisfactorily dealt with. Our research work consists of finding those problems that have the right combination of interest and importance and to the solution of which we can make a significant contribution.

One that is at the top of the list in my group right now is the puzzle of the termination shock of the solar wind. There is a controversy over observations from the Voyager 1 spacecraft at some 90 AU from the sun. One experimental group believes that the data indicate that the shock has been crossed, which would be a major milestone, but other experimental groups disagree. We are attempting to make a consistent physical picture of what is going on. See this NASA press release from November 2003.

A non-technical overview of this, "Voyager goes close to the edge", was published in the British journal Physics World in January of 2004. It's a bit later than the NASA release and hence more up-to-date, and also perhaps adds a bit more perspective.

Dr. J.R. Jokipii
(September 2004)

You might want to start looking for ideas on the American Physical Society's "Careers in Physics" page.

Dr. Louis Barbier and Ms. Beth Barbier
(May 2001)

In high school, physics, math, and computer programming are all useful. I personally also found that the high school typing class I took was well worth it. At the undergraduate level, again physics, math, computers, and astronomy. Most specialization (radio astronomy, for example) is done in graduate school, but if you pick an undergraduate school that has an active radio astronomy program, you can likely pick up useful work-study experience. For more information, you can look at the Imagine the Universe! web site.

Dr. Eric Christian

This is well beyond our area of expertise. You might see if you can talk to an informal educator at a local science museum or planetarium. They generally have a fair amount of experience with presentations to this age group.

Beth Barbier
(October 2003)

Quite simply, spherical trigonometry is the study of triangles on a spherical surface - that is, the relationship among the arc lengths, and the spherical angles between the arcs. If the size of the triangle (its area) is small compared to the area of the sphere, then spherical trigonometry will reduce to plane trigonometry.

In astronomy, one must use spherical trigonometry to determine the positions of the stars (or other objects) and their relationships to one another.

Dr. Louis Barbier
(March 2002)

You will want to contact the Astronaut Appearances Offices. The only cost is for the expenses associated with the appearance (transportation, accomodations, meals, etc.), but it looks like the competition is stiff for the astronauts' time.

There are other types of speakers you might have a better chance of obtaining - check out the NASA Speakers Bureau. I hope you are able to find someone to meet your needs.

Beth Barbier
(April 2005)

The NASA Jobs web site covers your question.

Beth Barbier

Unfortunately, we do not have time to answer the many interview requests that we receive. You might want to read the interview with Dr. Louis Barbier. Many of your questions are answered there.

Beth Barbier
(February 2003)

I'm currently working on several different projects, all at different stages of development. One of the main projects is the Advanced Composition Explorer (ACE), which is a small spacecraft that is currently 1 million miles from the Earth measuring particles from the Sun, interstellar space, and the Galaxy. Math is required at nearly every step of the process, from designing, building, testing, launching, and analyzing the data. Mostly it's algebra (solving several equations for un known quantities) or calculus (differential equations). I don't really have a "favorite" math, I consider math to be a tool and use whatever math is needed.

Dr. Eric Christian

The best place to start would be on his own home page.

Beth Barbier
(December 2000)

You are not too old to start college and become a physicist. But it is not going to be easy. You'll probably need nine or ten years of university (4 to get a bachelors degree and at least five more to get a PhD), during which time you won't be earning very much. My suggestion is to find a college that will accept you, and then transfer to a better college after a few years if the first is not good enough (especially in Physics). Once you get your bachelors degree, your high school grades are irrelavant. Good luck!

Dr. Eric Christian

The StarChild website is designed for students under age 14. I can also suggest that you check the list of links on ACE's Cosmicopia website, especially the specific science topics, spacecraft, and space pictures sections. You might find something there that interests him. And of course, there's always the very popular Astronomy Picture of the Day. I hope your son will find something in this list that draws him in.

Beth Barbier
(October 2005)

ACE has worked with teachers to develop web-based education briefs and classroom activities based on ACE science. You can find several of these on our "Great Links" page, with at least one aimed at grades 5-8. ACE hasn't developed educational materials for earlier grades due the complexity of the subject -- once you break things down to a 1st grade level, it doesn't matter whether it's based on ACE science or SOHO science, for example. Those at this URL were composed last year.

For more activities aimed at elementary students, though not about ACE science, you might want to check out StarChild site.

You might find Cosmicopia, based on ACE and other cosmic ray mission science, helpful as well. Though this site is aimed at a middle and high school level, it might be helpful to you for background material, and there are several individual features that you might be able to use in some of your classes. Our most popular feature is the "Ask Us" service. Send in your questions on ACE, and any other subjects in our area of expertise, and we will be happy to answer them.

Other features of Cosmicopia you might use include the glossary, the history page, and the space weather page (very big in the news right now, and very understandable for elementary students). All of our subject pages, such as the space weather page, have links to related educational activities and web news articles.

Of course, you can always check out the ACE Project Page.

We'd love to hear any suggestions you have about how to improve the ACE-developed activities and web sites, once you've taken advantage of them.

Beth Barbier

Space Camp is available in Alabama, California, and Florida. They have a web site. You can also check out the NASA Education Program web page, or check with your school guidance counselor, county or state school board, or local university to see if any programs there might meet your needs and budget. Best of luck to you!

Beth Barbier

I did a Google search for [satellite imagery earth "real time"], which returned 1.5 million sites. Looking through the first page, there are several that look promising:

• Earth and Moon Viewer
• Satellite Composite Images from the University of Wisconsin - Madison
• Google Earth

Beth Barbier
(October 2005)

Jupiter, Saturn, and Venus are all brighter than almost all the stars you can see (except for the Sun), and Mercury and Mars are both easily visible with the naked eye. But there is an inherent difference between stars and planets that make stars visible across the many light years of space. Stars glow with their own light, while planets only reflect the light coming from their star. Nuclear fire (fusion) makes stars extremely hot and bright. That's what makes them stars instead of just large planets.

An analogy is a match in a dark room. You can easily see the lit match anywhere in the room, but in order to read, you need to have both the match (to supply the light) and what you're reading very close. You read by seeing reflected light.

Dr. Eric Christian
(June 2000)

There are lots of satellites in orbit that are easy to see just after sunset (they are still in sunlight even though the sky is dark). These may well be what you're seeing.

Dr. Eric Christian
(April 2002)

A good library should have books on amateur astronomy that will show you how to build your own telescope. It is far too extensive a subject to cover in any web page that I know of.

Dr. Eric Christian

In my physics class, we want to construct a large telescope out of an old satellite dish. We want to put many small mirrors on the satellite dish to make a lens. Will the small mirrors cause the light waves to cancel each other out and ruin the clarity of our image? Do you have any suggestions on how we should do this?

An interesting idea. There are a few multi-mirror research-grade telescopes in existence, such as the Keck and Hobby-Eberly telescopes. The mirror segments in these scopes are formed to very high tolerances both in terms of surface smoothness and radius of curvature. Your job is complicated by the need to have curved mirror surfaces all focusing at the exact same focal plane. A number of flat mirrors placed on your satellite dish will approximate a spherical or, more likely, a parabolic shape but will likely not have the needed accurately curved surface. Remember, in a single mirror telescope, the surface is figured to less than 1/4 wavelength of red light (also called "quarter wave surface accuracy"). Additionally, radio telescopes generally have fairly low focal ratios. This makes the requirements for mirror figuring even more stringent.

Radio telescopes operate just fine with rough, mesh surfaces because the wavelengths of light they are sensitive to are relatively long. Optical telescopes, sensitive to much shorter wavelengths, require much higher tolerances. So I am afraid it would be difficult for you to build a sufficiently accurate composite surface. However, have you thought of trying to renovate this dish as a radio telescope? There are, I believe, commercial kits that provide detectors sensitive to 21 cm hydrogen emission. Check Sky and Telescope ads (in the back of the magazine).

Dr. Lou Mayo, Planetary Scientist and "Guest Answerer" for Cosmicopia
(April 2002)

Airplanes need air in order to keep from falling. Flying requires the atmosphere to generate lift (there is more force on the bottom of a wing than on the top). But as you get higher, the atmosphere gets thinner and the lift decreases. You can compensate by going faster, but at some point the drag of the wings (the friction between the wings and the atmosphere) is worse than the lift they generate, at which point it's better to just "rocket" your way out of the atmosphere.

Most of the distance between the Earth and the Moon has no air. A rocket will work even in space. There have been suggestions to build a combination of airplane and rocket, which would fly like a plane to the edge of the atmosphere (the area of air around the Earth) and then would use rockets to go the rest of the way to the Moon. But it hasn't been done yet.

Dr. Eric Christian
(July 2001)

Sound waves are formed when a medium (like air or water) is compressed, and the speed of sound depends on what medium it is traveling through. Interplanetary space isn't a perfect vacuum - it is composed of tiny dust particles, solar wind particles (mostly electrons and protons), and cosmic rays. Near Earth's orbit, there are about five atoms per cubic centimeter in interplanetary space - compare this to the density on Earth's surface, which is 10¹⁹ atoms per cubic centimeter (that's a 1 followed by 19 zeros!). Technically speaking, sound waves can exist in outer space since it isn't a perfect vacuum, although the frequencies are far too low for human ears to hear them (so if you have ever seen a sci-fi movie where spacecraft explodes with a loud boom, that is inaccurate!).

You are correct that the speed of sound is much larger in space than on Earth. On Earth, the speed of sound is about 740 miles per hour, or mph, at sea level, while in interplanetary space it is roughly 900,000 mph near Earth's orbit. Spacecraft generally travel much slower than the speed of sound in space. For example, the Cassini spacecraft that traveled to Saturn sped along "only" at about 11,700 mph.

The main problem with using a supersonic jet to travel in space is that jets and other aircraft used on Earth are not designed for space travel. A jet provides thrust by taking in surrounding air molecules in the atmosphere, passing them through its engine, and expelling the air at high speeds. In space, the density of gas is far too low for intake, and jet engines would not be able to provide thrust. On the other hand, a simple rocket engine provides thrust by burning fuel to create and accelerate high-pressure gas to a high velocity, which is then ejected. By Newton's 3rd Law of Motion, every action has an equal and opposite reaction, so the rocket moves in the opposite direction of the ejected gas. These engines do not need surrounding air to provide thrust like jet engines do.

Dr. Nick Sterling
(February 2007)

Your question is beyond our area of expertise, but you might want to check out JPL's Basics of Space Flight.

Beth Barbier

The sun is a source of harmful ultraviolet (UV) radiation and high-energy charged particles (such as electrons and protons, in the solar wind). High-energy cosmic ray particles are also potentially harmful to humans. Earth's atmosphere and magnetic field block or deflect the worst of this from us on Earth, but it is a different story for spacecraft and astronauts.

UV radiation is not so dangerous for space travel, since spacesuits and spacecraft carrying humans are designed to block harmful radiation. It is the high-energy particles from the solar wind that can wreak havoc. The danger from these particles is usually not serious unless the spacecraft is caught in a solar storm (such as a coronal mass ejection), when the intensity of charged particles (and radiation dosage) are higher than normal.

As you surmised, electrical charging of spacecraft is one of the effects that can be caused by the sun's radiation or solar wind. Differential charging occurs when sunlight or charged particles cause different parts of the surface of a spacecraft to occur and can cause damage to electrical components. Bulk charging can occur when high-energy particles (mostly electrons) penetrate the surface of the spacecraft and charge the interior of the spacecraft. This again can lead to electrical discharge that is harmful to electrical components.

The effects of high-energy particles on humans is worse. High-energy electrons or protons lead to chromosome damage and cancer. If the radiation dosage is high enough, radiation sickness or death can occur.

Now for the good news. Spacecraft charging has been well-studied, and spacecraft are designed to mitigate differential and bulk charging effects. Moreover, aside from the Apollo flights to the moon, astronauts have not in general received dangerous exposure to solar wind particles. This is because spacecraft such as the International Space Station are within the protective shield of the Earth's magnetic field.

On the other hand, a manned flight to Mars could pose greater risks. Since coronal mass ejections can be detected by the light that they emit before the high-energy particles reach the spacecraft, this gives astronauts time to take protective measures. For example, they could take cover in a "storm shelter" at the center of the ship, surrounded by vats of water that shield the worst of the solar wind particles. Astronauts also routinely carry dosimeters with them, indicating the level of radiation to which they are exposed.

Dr. Nick Sterling
(May 2007)

This is not our area of expertise, but please see this NASA GSFC NSSDC web page for a description that will answer your question.

Dr. Louis Barbier
(July 2000)

There is no orbit that stays 100 m above the same point on the equator. If you lifted a rocket up, it would fall back down. Because of the 1000 km/hour sideways motion, the rocket would come down at the same place it took off from, although both the rocket and the ground would have moved. Stable orbits 100 m up need to orbit the Earth in less than 90 minutes, so you would need to rocket off in some direction at an even higher speed. Orbits usually go in the direction the Earth is spinning so that you start off with 1000 km/hour that you don't need rocket fuel for. At that height (100 m), the air resistance would cause your orbit to decay pretty quickly, and besides, you would probably hit something before you got once around.

Dr. Eric Christian
(May 2001)

This is a common misconception. In reality, the rocket blast doesn't need anything to push against. The blast is accelerated in one direction, and, since every force has an equal and opposite force, the rocket is accelerated in the other direction. It works in space just fine.

Dr. Eric Christian
(January 2002)

When a rocket is fired in space, what does the thrust push against? I've been wondering about this for years!

Rockets don't need to push against anything, so they work both in the atmosphere and in space. The rocket exhaust shoots out the back at high velocity, and in order to conserve momentum, the rocket accelerates in the other direction. This has a couple of major consequences for spaceflight. One is that the efficiency of a rocket is determined by the velocity of its exhaust, and it's very hard to get it faster with chemical rockets (which is why new rockets aren't much more efficient than the first generation rockets). The other consequence is that you need to use up a lot of mass of rocket fuel (going into exhaust), which is why rockets to orbit have to be so big.

Dr. Eric Christian
(November 2002)

Yes, the falling white material is ice. Some of the fuel used (liguid oxygen and liquid hydrogen) is kept very cold and in the moist Florida atmosphere, ice condenses on the outside of the rocket and falls off at launch. It is expected and doesn't do any harm.

Dr. Eric Christian
(Febuary 2002)

This is well outside our area of expertise, but there is a History of On-Orbit Satellite Fragmentations on the NASA Orbital Debris Program Office website.

Beth Barbier
(May 2005)

A geosynchronous satellite is one that goes around the Earth exactly once per day. If its orbit is over the equator, it will appear to be in the same spot in the sky all the time, which makes it easy to point antennas at it. The time a satellite takes to orbit the Earth depends upon its altitude, and so all geosynchronous orbits are at 35,786 km. Arthur C. Clarke (a famous science fiction author) is credited with being the first one to figure out this orbit.

Dr. Eric Christian
(June 2001)

Check out the Space Shuttle web site.

Beth Barbier

The detailed answer to your question is beyond our area of expertise, but you might want to check out this site at NASA GRC on what keeps objects in orbit.

Beth Barbier
(March 2000)

Earth's gravity must still have a large magnitude at the altitude of shuttle flights. How do they stay up there, and why are they weightless?

You are correct the Earth's gravity is still quite strong, so calling it "zero gravity" is incorrect (although it's frequently used). The reason the shuttle stays up there is because it has enough velocity to be in orbit around the Earth. In an orbit, you are always falling towards the Earth, but your velocity makes sure you never actually hit the Earth. When an object is freely falling (no resistance due to atmosphere or whatever), it is "weightless" or in "free fall".

Dr. Eric Christian
(October 2000)

Why does a spacecraft need to go 17,000 mph to re-enter the Earth's atmosphere? Why can't it just settle (float) slowly back into the atmosphere, such as with the aid of a parachute, or hover? It seems to me that if a craft was able to just settle into the atmosphere there would be very little friction rather than being a 17,000 mph fireball.

A spacecraft needs to be traveling at about 17,000 mph because that's the velocity of everything in a low orbit around the Earth. The large shuttle or rocket engines are needed to get the spacecraft up to that speed in the first place. In order to settle slowly back down, you need to get rid of all of that speed. The two ways of doing that are to use large rocket engines again (using just as much fuel as you used to get up there), or to come in quickly and use the atmosphere to slow you down with friction. It doesn't make sense to use fuel, because you would need even more fuel to get your landing fuel into orbit, so rockets would have to be 10 times bigger and more expensive. It is easier (and much cheaper) to protect against atmospheric friction.

Dr. Eric Christian

How do you increase the inertia when you add more mass to the space station? My teacher said that the amount of gravity holding it in orbit would increase if you added more mass. One of the only ways to prevent it from crashing into Earth would be to increase it's inertia, or speed. How do you increase it's inertia?

The speed that an object is moving determines what orbit it is in. If you add more speed, you would change the orbit. The space shuttle uses fuel to "match orbit" with the International Space Station, or in other words, it accelerates until it is at the same speed as the ISS. Then mass can be moved to, and mounted on, the space station and there is no problem. It's all moving at the same speed and stays in the same orbit. The shuttle then uses more fuel to slow down and fall out of orbit so it can land.

Dr. Eric Christian
(March 2001)

You are correct that fire requires oxygen, and that there is no oxygen in space. The Shuttle has a very complicated propulsion system, with 3 main engines (used for liftoff), 38 primary reaction control system engines, 6 secondary ("vernier") reaction control system engines, and 2 orbital maneuvering system (OMS) engines. These engines carry their own fuel with them - helium, nitrogen, and other gases are stored and used as needed by the different engines. I suggest you visit these two Web sites for more detailed information on the Space Shuttle:

• Space Transportation System and
• Orbital Maneuvering System

Dr. Louis Barbier

A guess from the reader:
In order to sail closer to the Sun, maybe the solar sail could be shaped like an air foil and the craft continuously tack like a boat does?

I had a professor in graduate school raise this question, and all of us who were sailors said the same thing as you. We were all wrong.

A sailboat has a keel, or a centerboard, that keeps it from slipping sideways. If you sail a small boat into the wind and pull the center board, it immediately slips sideways, and forward motion is lost. What is available in interplanetary space for a keel to grab onto?

So you use a solar sail to eliminate angular momentum -- that orbital motion associated with launching a spacecraft from the Earth, with the motion of the Earth imparted to the spacecraft. You tilt the solar sail at an angle to the wind so that it pushes the spacecraft against the orbital motion of the Earth, and slowly the spacecraft falls sunward. The spacecraft becomes a falling stone. So much for the romance of sail.

If you conserve angular momentum and reduce the distance from the spacecraft to the Sun, the orbital speed increases in order to conserve angular momentum. This carries the spacecraft back away from the Sun. In order to move sunward, you need to eliminate angular momentum so you can fall into the Sun.

Dr. Charles Smith
(January 2005)

The terminal velocity (the fastest speed it could reach) for something as large as a shuttle in water is certainly less than 100 kph, which is small compared to the velocity the shuttle attains. The added complexity for all of the systems required to launch this way makes this system much, much, much less efficient than the current method (a little extra fuel).

Dr. Eric Christian

I would ideally like to construct a craft capable of lifting off the ground with an ion engine instead, but I am under the understanding that the thrust provided by an ion engine is not enough to operate in a normal atmosphere. I don't know if I could even construct an ion engine without xenon gas that would work like I hoped, even in a vacuum. I was contemplating building a vacuum chamber to operate one in, but I have not really done enough research to conclude if that is possible for me either. Could you give me some advice?

I'm glad to hear that you're interested in physics/astrophysics. Ion propulsion is certainly an interesting topic, but I'm afraid its limitations just make it unsuitable for an impressive project here on Earth.

The atmosphere isn't the biggest problem. It's the 1 G of acceleration that we get from the Earth. In order to construct a craft capable of lifting off the ground, you need to generate more than 1 G, and ion engines only accelerate a fraction of that.

And I'm afraid that building a vacuum chamber large enough for testing is quite a task. It would need to withstand the differential pressure of 1 atmosphere, which would be thousands of pounds of force.

I could think of a science fair project that measured the velocity and mass loss of an ion gun and a chemical rocket (the engine for a model rocket, say) and compared the efficiencies.

Dr. Eric Christian
(June 2008)

If you can store antimatter (presumably in something like a magnetic bottle), then it can make an excellent fuel. It is a BIG step up from chemical burning of fuel, because the efficiency is much, much higher. Most of the mass of chemical fuel is wasted in the propulsion stream, but that is mass that you have to take with you (and also accelerate, which takes more fuel). But you only need a small amount of antimatter to accomplish the same thing. If you are going to use matter/antimatter annihilation, the easiest way to grab the resulting energy is as heat, which is why the ideas for antimatter propulsion are along these lines. If you control the amount of antimatter that is annihilated, you shouldn't have any trouble, but if your entire supply goes up at once (the standard science fiction "breach of the containment field") you would probably lose the ship.

Matter/antimatter annihilation is different than nuclear fission or nuclear fusion, by the way.

Dr. Eric Christian

It's the dream of many people to find a faster-than-light method of transportation so that we can travel to the stars. At the current time, the laws of physics, as we understand them, don't allow this. How does one search for an undiscovered energy source? Plus, the amount of energy required appears to be infinite. Anti-gravitinos have not even been discovered, so harnessing them is also beyond us at the current time.

This doesn't mean we should stop dreaming, however. You can see what NASA is investigating along the lines of "Warp Drive" at this NASA LeRC site.

Dr. Eric Christian

This is not as easy as it sounds. As the rocket's velocity increased with each gravity assist, it would get harder and harder to completely change direction so that it would be headed back at the other black hole. You might be able to do it by getting closer to the black hole with each pass (which doesn't sound too different from spiralling in to your death!) Any passengers would probably end up as red goo as your rocket undertakes thousands of G turns around the black hole, and at some point your electronics and rocket would fail as well.

Dr. Eric Christian
(April 2000)

You don't say what grade you're in, and this is a complicated subject. If you are an advanced high school student or a college student, check out the University of Wisconson web page on this subject.

Dr. Louis Barbier

This web site at the University of Wisconsin might help get you started. The equations you want aren't there, but the information is sufficient to derive the equations. The key to gravity assist is that the spacecraft undergoes a hyperbola in the planet's reference frame (it enters and leaves with the same velocity), but since the planet is moving, you can gain the speed of the planet times some geometrical factors.

Dr. Eric Christian

Yes, the astronauts left the flag there (as well as a bunch of equipment).

Dr. Eric Christian
(April 2002)

The timescale for erosion (which is only due to micrometeorites and solar wind impact) is millions of years, so the answer is yes. All of the astronauts footprints are still there.

Dr. Eric Christian
(April 2002)

The only complete dropouts that I know of were caused by the large discontinuity called the Moon, which visible light, radio, or other electromagnetic waves can't pass through. If there were electromagnetic discontinuities the way you propose, they would be easy to see as completely black patches against the background stars. There is NO evidence of such things.

Dr. Eric Christian

Your question is beyond our area of expertise or interest, but you can look at The Great Moon Hoax from Science@NASA.

Another site, with point-by-point explanations, is on Phil Plait's Bad Astronomy.

I hope this helps you to understand that NASA did land men on the Moon in the 1960s and 1970s.

Beth Barbier
(August 2001)

Well I'm no expert on communications, but I think it has to do with the fact that the shuttle is orbitting the Earth pretty quickly (one orbit every 90 minutes). TV stations use satellites in geo-stationary orbit, where they appear to be in the same place in the sky all the time. It is easy to point an antenna to them. With the shuttle, you have to move the direction the antenna is pointing pretty quickly and even change ground stations or satellite uplinks completely fairly often. Which isn't to say that it couldn't be better if NASA decided to spend more money on it. But I don't think that's the highest priority.

Dr. Eric Christian

The explanation is simply that they WERE moving slowly. The combination of caution and very bulky spacesuits makes quick movements difficult and unwise. In zero-gravity (even without spacesuits), the astronauts tend to move slowly, because quick movements can send them spinning.

Dr. Eric Christian

The space shuttle is not designed to go to the Moon, only to orbit the Earth. The last Apollo mission flew in 1972, and it took 3 to 4 days for the Apollo spacecraft that went there to get to the Moon. You can read more about the Apollo missions at this NASA GSFC site.

Beth Barbier
(April 2000)

The Moon has gravity, about 1/6th of that on Earth. so he jumps higher and comes down slower than he would on Earth.

Beth Barbier
(March 2000)

Gravity falls off like the distance squared. The Earth is about 6324 km in diameter and the International Space Station is about 440 km up (the actual altitude varies with time). So an astronaut feels

1G * (6324 * 6324) / (6764 * 6764) = .874 G

Dr. Eric Christian
(May 2002)

This is beyond our area of interest, but it has been covered by the Imagine the Universe! Web site.

Beth Barbier
(March 2000)

This is not our area of expertise, but you might want to check out the space station page on combustion in microgravity.

Beth Barbier

Plants tend to grow toward the light source (the Sun). You can test this by growing a plant off to one side of a window. It will angle toward the window instead of growing straight up. Animals (and people) have a shape that comes from their genetic code. A human or frog (frogs have been raised in zero gravity), will still have the same body shape, just as a child who grows up in bed, never standing up, will still have a human body shape. Zero gravity will certainly affect a child's bone strength, and they may grow to slightly different proportions than they would on Earth, but the differences will not be large.

Dr. Eric Christian

A REM (Roentgen Equivalent for Man) is a dose equivalent unit which tries to quantify the probable damage in biological tissue for a given dose. It is integrated over time, that is, the limit is a lifetime dose limit.

Dr. Louis Barbier

Currently it is not possible to clone human beings. That will probably change in the future. But I fail to see why cloning is necessary. The natural way of making babies is considerably easier to implement, and you can always send frozen sperm/eggs/fertilized eggs, and then have your machine grow them, if all the humans are dead. Cloning, as with this system, still leaves you with the problem of growing and training the new humans.

As far as the plants go, there are more efficient chemical methods for replacing oxygen and food, although plants would make a good supplement. The problem with relying solely on plants is that a large amount is required to fully resupply a small group of humans with air and food.

For fuel, a nuclear reactor is probably the best bet with current technology.

Dr. Eric Christian

Spacesuits are both air-conditioned and heated. Design in space is pretty complicated. If you are in direct sunlight, you can get very hot; if you are in shade, you can get very cold. The Earth is also a factor, since it's a lot warmer than the blackness of space (which is at 2.7 degrees Kelvin) and takes up a lot of the sky when you're in low Earth orbit. The spacesuits are white because that is the color that minimizes the difference between sunlight and shade.

Dr. Eric Christian

The 12G limit can be gotten around somewhat with a water tank and G-Suit, but it wouldn't be comfortable. And it is unnecessary, because a 1G constant acceleration would get you to high velocities pretty quick (days). But we currently don't have any way to continuously generate 1G acceleration. The other limit (300 miles/sec) just doesn't exist. Even if you were moving at a steady 30,000 miles/sec, your brain would work just fine.

Dr. Eric Christian

I find all of these to be unreasonable, I believe blood has far too much solute to boil even at 0 atmospheres, and that the human body has enough structural integrity to stay together, although I'm sure some blood would spew, possibly from the eyes and other orifices and all air would be forced from the lungs. Yet I consider the most probable cause of death to be a heart attack, caused by the heart's inability to compress at extremely low external pressures. Which one of us is most correct?

This question has been answered quite well on our sister site Imagine the Universe!.

The only thing I'll add is that the swelling of the skin will essentially be a whole body "hickie", which doesn't sound very comfortable to me.

Dr. Eric Christian

Astronauts are always exposed to at least some level of cosmic radiation, and the Apollo astronauts, the only ones who travelled beyond the protection of the Earth's magnetic field, received the most exposure. They are made aware of the risk (one of many) and there doesn't seem to be a large increase in their chance of getting cancer, but the statistics are small. This is a real worry for manned missions to Mars, where the astronauts may be out there for more than a year. They will probably need some sort of shielding, but the mission hasn't been designed yet.

Dr. Eric Christian
(May 2002)

The radiation received in orbit consists of two main types: x-rays and particles. What you have access to at your dentist's office is, of course, x-rays. The typical dose from your dental x-ray is many, many orders of magnitude greater than what is seen in orbit, even during very large solar flares. Incidentally there was a large solar flare the same day Mission 102 (Discovery) launched (April 8, 2001).

I don't know enough about dental x-ray machines to know how "low" they can go in dose, but probably not enough to mimic a space exposure. Even if they could it may not help, because standard 35 mm film is just not very sensitive to x-rays: it is just too thin. By contrast, the film your dentist uses is a special film that is much thicker, as are the films scientists put in space to study x-rays too.

Your film is much more sensitive to the particle radiations, I'm afraid.

Dr. Louis Barbier and Beth Barbier
(April 2001)

I am not an expert on manned space travel. There is a NASA web page for space flight that is much better suited for these questions here: Space Flight Questions and Answers.

Dr. Eric Christian

This question is beyond our expertise. NASA has a website with information about Columbia.

Beth Barbier
(February 2003)

The closest spacecraft have been to date was about 0.29 AU; these were the Helios 1 and 2 spacecraft in the 1970s.

We are working on a NASA mission called Solar Probe Plus (currently scheduled for launch in 2018) that will go to 0.05 AU. It will use multiple gravity assists at Venus to lose enough energy to get close to the sun. I am one of many co-investigators. The spacecraft will be shielded by a carbon heat shield that will reach temperatures of several thousand degrees. The instruments behind the shield should remain at ~room temperature or below.

Dr. Richard Mewaldt
(January 2012)

One never knows, but that shouldn't stop you from pursuing a career at NASA. You might be able to help make it happen!

Dr. Louis Barbier and Beth Barbier
(March 2000)

This is a question for the SOHO mission. Check out the FAQ page for "Dr. SOHO." If you don't find your answer there, you have an opportunity to send it in to Dr. SOHO.

Beth Barbier
(October 2005)

The cost to build ACE, launch it, and get it operational on orbit was about $107 million. There is also a cost to keep it runing. That was originally about $8 million per year but is now close to $4 million a year.

Dr. Eric Christian
(February 2011)

SOHO and ACE, which are at the Earth-Sun L1 libration point, are not in Keplerian orbits. Kepler's laws work for what is call "two-body" problems, i.e. something (a spacecraft, moon, or planet) orbiting another something (planet or sun). If the Earth weren't there, SOHO would have a "year" of less than an Earth year, and so would race ahead of the Earth. The Earth's gravity "holds it back", and L1 is the special point where the gravitational attraction makes the orbit around the Sun exactly one year. This is a "three-body" problem (actually four body, some of SOHO and ACE's fuel is used to compensate for the fluctuations caused by the Moon).

There's a very nice description of the Lagrange points on the WMAP web site. WMAP will be heading out to L2.

Dr. Eric Christian and Beth Barbier

The L1 Earth-Sun libration point is essentially right on the Earth-Sun line, and so, from any point on Earth, you just point at the Sun. The Sun's radio noise will drown out anything right at L1, which is why spacecraft like ACE and SOHO don't sit at L1, but orbit L1 in what is called a halo orbit. This way the spacecraft are 5 - 15 degrees away from the Sun and its radio noise.

Dr. Eric Christian

Although there is an L1 point for the Earth-Moon system, ACE (and SOHO) orbit the Earth-Sun L1 point, which is well beyond the orbit of the Moon. The other thing to realize is that ACE is not in what is called a Keplerian orbit (one thing revolving around another), it is affected by the gravity of both the Earth and the Sun, and the "three-body problem" is not easy. Also, the net gravitational force of the Earth and Sun are not zero. If they were, ACE wouldn't revolve around the Sun, the net force is still towards the Sun. L1 is a point where, if you pull away in any direction perpendicular to the line between the Earth and the Sun, the net force will pull you back in the direction of L1. In these directions, L1 is a gravitational valley. Along the Earth-Sun line, it is gravitationally a hill. If you move closer to the Sun, the force is towards the Sun, etc. That is why ACE's orbit is only semi-stable, and is one of the reasons we need fuel to stay there. We orbit L1 in the plane perpendicular to the Earth-Sun line. Since L1 revolves around the Sun, we are really doing a sine-wave pattern in orbit about the Sun. We don't sit right at L1 (which would be more stable) because the very bright Sun would be directly behind our radio transmitter, as seen from Earth. It would be like trying to see a firefly with a spotlight pointed at you.

Dr. Eric Christian
(November 2001)

The balloon you saw was almost certainly carrying the BESS experiment. BESS is a Japanese/American experiment to look for antimater in cosmic radiation. There is a write up on it at this link.

Dr. Eric Christian

NASA has a Java applet, called Skywatch, that calculates when you can see the space station and other objects.

Dr. Eric Christian
(December 2000)

I think it is highly unlikely there will be a mission to Mars in your lifetime, and almost certainly not in the next 20 years. This is not a NASA priority right now, nor does it seem to be a priority for the nation as a whole.

Many non-trivial issues would need to be solved for such a trip - including protection from cosmic radiation, and how to prevent or repair calcium loss from bones, which occurs in microgravity environments.

Dr. Eric Christian
(November 2002)

When are they planning to send the first group of people to Mars? And that is four people right?

Although there are people looking at what would be required in a manned trip to Mars, it will certainly be decades in the future, and the number of astronauts has not been decided yet.

Dr. Eric Christian

The Mariner series of space probes went to both Mars and Venus. Since you ask about Mars, Mariner IV was launched in 1964 and took almost 3/4 of a year (228 days) to reach its destination. Mars is about 141,690,000 miles from the Sun.

Dr. Louis Barbier

The Voyager spacecraft are still returning lots of information about the Sun's magnetic field and energetic particles in the distant solar system.

I have no idea of how much money Voyager currently costs, but it is almost certainly cheaper than sending a newer spacecraft out that far.

There is no UFO coverup. As I've said to other similar questions, there are too many of us who would rebel if such information was really being kept from the public.

The Voyagers will almost certainly never be picked up. Space is just too big, and they're too small.

Dr. Eric Christian

There are three reasons Voyager 1 passed Pioneer 10. The first is that Voyager 1 left the Earth at a higher speed, probably due to the newer rocket (Titan-Centaur instead of Atlas-Centaur). The second is that the gravitational assists at Jupiter and Saturn increased Voyager 1's speed more. Voyager 1, in order to get closer views of Saturn's moon Titan, got in closer to Saturn, which gives a higher change in velocity. And third, Voyager 1 continued pretty much straight out of the Solar System, whereas Pioneer 10 sort of cut back across the Solar System after Saturn.

Dr. Eric Christian
(March 2004)

The first "Interstellar Probe" won't get anywhere near Alpha Centauri, it will just get out of the solar system. It will fly down closer to the Sun than the Earth is and use a solar sail to pick up speed. It will then jettison the sail and coast out of the solar system. For more information, you can check their web site.

Dr. Eric Christian
(November 2000)

It seems to us that the best place to start looking would be the Hubble web site.

Beth Barbier

Your question is beyond our area of expertise. The best place to go is probably the SETI (Search for Extraterrestrial Intelligence) web site.

Beth Barbier
(March 2005)

This isn't our area of expertise, but you can find information about this at the SETI Institute web site.

Beth Barbier

Your question is really outside of our area of expertise, but there's a good Web site on the subject at Scientific American Frontiers. You'll see that N is the number of intelligent civilizations able to communicate within our own Galaxy.

Beth Barbier
(March 2000)

Although some versions of the Drake equation may not include it, in addition to a planet being in the habitable zone around its host star, that star must be in a habitable zone of the galaxy. If a star is born too close to the center of the galaxy, there may be too much supernova activity for a planet to evolve and sustain life. Likewise, if the star is born farther out from the center of its galaxy, there may have not been enough supernova activity before the star was born to enrich the environment with enough carbon, nitrogen and oxygen to facilitate life as we know it.

I know of no definite value for the fraction of a galaxy in this inhabitable zone, but it is most likely quite small with large uncertainties. As an aside, 90% of elemental matter is hydrogen, 9% helium and less than 1% heavier elements, on average. These values may vary only slightly as you get farther away from the center of the galaxy.

A good book that you might be interested in, if you have not already read it, is "Rare Earth: Why Complex Life is Uncommon in the Universe" by Ward and Brownlee.

Lauren Scott
(January 2005)

Your question is beyond our area of expertise, but there's a web page on this at the Johnson Space Center.

Beth Barbier

The idea isn't silly, but there are several problems with it.

One is that this orbital debris is typically moving very fast, but is only weakly magnetized, and a strong enough magnetic field to deflect it would take a large amount of power.

Two is that you can remotely determine something about the object's composition, but not its magnetic polarity.

And the third, and most fatal, problem is that if you could determine the magnetic polarity of the oncoming object, and had the huge amount of power needed to switch the polarity of your large magnetic field, the object would flip over so that it was attracted to you, not repelled, and would be accelerated so that it hit you harder!

Dr. Eric Christian
(June 2000)